1. Field of the Invention
The present invention relates to molds and methods for manufacturing the same.
2. Description of the Related Art
In an injection molding machine of the related art, resin heated and melted in a heating cylinder is injected into a cavity of a mold apparatus under high pressure so that the cavity is filled with the molten resin. The molten resin is then cooled and solidified so as to obtain a molded article.
The injection molding machine includes an injection apparatus, a mold clamping apparatus and the mold apparatus. The mold apparatus is provided with a stationary mold and a movable mold. The mold clamping apparatus includes a stationary platen, a movable platen, a motor for mold clamping, and others. The movable platen is advanced and retracted against the stationary platen by driving a motor for mold clamping, so as to perform mold closing, mold clamping and mold opening.
The injection apparatus includes a heating cylinder and an injection nozzle. The resin supplied from a hopper is heated and melted by the heating cylinder. The molten resin is injected by the injection nozzle. A screw is disposed inside the heating cylinder so that the screw can be rotated about an axis and can be advanced and retracted. The screw is advanced by driving a motor for injection so that the resin is injected by the injection nozzle. The screw is rotated by driving a motor for metering and thereby the screw is retracted and the resin is metered.
Meanwhile, in a case where precision parts such as a disk shaped substrate, a lens made of plastic, and the like are molded as a molded article, the quality of the molded article is determined based on the cavity space of the mold apparatus. Because of this, the mold apparatus is required to have high precision for various measurements of the mold apparatus. Hence, the mold is manufactured by the following method.
It is noted that the molds (the stationary mold and the movable mold) forming the mold apparatus have not only the mold bodies such as the cavity and a core but also a mold insert body or a mold core body provided with the mold body if necessary. Hence, in this specification, the “mold” is a general term of the mold body, the mold core body, and the mold insert body. In a case where a disk shaped substrate is molded, a stamper is used as the mold insert body.
FIG. 1 is a vertical cross-sectional view of a mold of the related art.
Referring to FIG. 1, the mold insert body 11 forming a part of the mold includes a mold prototype 12 and a nickel-phosphorus plating layer 13. The mold prototype 12 is a prototype of the mold insert body 11.
The mold insert body 11 is manufactured by the following steps. In the first step, a base material blank made of steel material such as SKD 61 including chrome of approximately 2–5% is formed. In the second step, rough processing is performed on the base material as having an error range of 20–200 [μm] so that the mold prototype 12 is formed. Next, in the third step, hardening and tempering are performed on the mold prototype 12.
In the fourth step, electroless nickel-phosphorus plating is performed on a mold surface S2 forming at least the cavity space of the mold prototype 12. As a result of this, a plating layer having a thickness of at least 100–200 [μm], namely the nickel-phosphorus plating layer 13, is formed.
In the fifth step, a heat treatment at a temperature of 300–400 degrees centigrade is performed, so that stress of the nickel-phosphorus plating layer 13 is removed and hardness (HRC) of 50–54 is set.
In the sixth step, external diameter processing is performed against the entire mold prototype 12 using a grindstone so that a reference plane is formed. After that, in the seventh step, rough configuration processing is performed on the nickel-phosphorus plating layer 13 by diamond bit cutting so that a cavity forming surface is produced. In the eighth step, a finishing process is performed on the nickel-phosphorus plating layer 13 of the cavity forming surface by diamond bit cutting so that the mold is finished.
In this case, the surface layer of the nickel-phosphorus plating layer 13 is amorphous. Therefore, as compared with a case where the finishing processing is performed on a part being in a crystalline state by the diamond bit cutting, a step due to the crystalline interface is not required to be included in the above mentioned steps. Hence, it is possible to manufacture the mold with high precision.
In a case where the disk shaped substrate as the molded article is molded by using the mold insert body 11 of the above mentioned related art, for example, the mold insert body 11 is set to the mold apparatus as a mold insert body (stamper) where a pattern of a hyperfine convex-concave is formed on the mold surface S2 thereof. The resin fills the cavity of the mold apparatus so that the pattern formed on the mold surface S2 is transferred to the resin. The resin is then cooled so that a prototype substrate is formed. At this time, the pattern is transferred to the prototype substrate.
And then, heat of the resin filling the cavity at the time of filling is transmitted to the mold prototype 12 via the nickel-phosphorus plating layer 13. In this case, the nickel-phosphorus plating layer 13 generally has a small thickness of 100–200 [μm]. Hence, the heat of the resin is transmitted to the mold prototype 12 immediately so that the temperature of the resin inside of the cavity-space is reduced rapidly. Accordingly, the pattern cannot be transferred to the resin precisely. As a result of this, it is not possible to form a disk shaped substrate with high precision and therefore quality of the molded article is degraded.
Furthermore, in the fourth step of the method for manufacturing the mold insert body 11, electroless nickel-phosphorus plating is performed on the surface forming at least the cavity of the mold prototype 12. However, work for electroless nickel-phosphorus plating not only is extremely troublesome but also takes a lot of time for manufacturing. Hence, the electroless nickel-phosphorus plating causes an increase of the manufacturing cost of the mold.
That is, in a case where the electroless nickel-phosphorus plating is performed, first a plating processing is performed on the mold prototype 12 in a plating bath after an ultrasonic cleaning, masking, striking treatment, or the like is performed. After that, the mold prototype 12 is cleaned. Thus, a lot of steps are necessary for electroless nickel-phosphorus plating.
Furthermore, in the above mentioned plating treatment, not only is the amount of the nickel-phosphorus adhering to the prototype mold 12 per unit time extremely small, but also the processing based on diamond bit cutting is required in the seventh and eighth steps. Because of this, since the nickel-phosphorus plating layer 13 is required to have a film thickness of at least 100–200 [μm], it takes an extremely long time to form the nickel-phosphorus plating layer 13.
In addition, not only is it easy for bubbles to enter the nickel-phosphorus plating layer 13 at the time of forming the nickel-phosphorus plating layer 13, but also it is easy for the nickel-phosphorus plating layer 13 to peel off and have a strain generated at the time of heat treatment of the nickel-phosphorus plating layer 13 in the fifth step. In the above mentioned case, it is not possible to manufacture the mold with a high precision, so that the yield rate becomes low.
Furthermore, in the plating treatment, there is a restriction of the composition of a plating liquid filling the plating bath. In addition, in a case where a steel material including chrome of approximately 13% is used as the base material blank, it is not possible to perform electroless plating on the base material blank and there is a restriction of the material of the base material blank. Therefore, it is difficult to manage manufacturing conditions of the mold.